In the first phase of this project the rapid sequence 51Chromium red cell survival test proved valuable in evaluating the clinical significance of irregular red cell antibodies. It is now a widely accepted laboratory procedure used primarily in situations when compatible blood cannot be found by usual in vitro serologic techniques. The project, therefore, now concentrates on those patients and blood donors who have poorly understood and incompletely characterized red cell antibodies detected in their serum. Characterization of the in vivo significance of these antibodies will allow more rational selection of blood for these people should transfusion be necessary in the future. A study of the significance of anti-H in the rare "Bombay" patient, and two papers describing unusual serologic problems solved by the 51Cr technique have been published. Studies continue on the significance of the Lewisb antibody and, in collaboration with the Red Cross, the clinical significance of certain variations observed in the direct antiglobulin test. Three distinct patterns of immune red cell destruction have been confirmed using 51Cr labelled cells. These patterns can be identified when the test is carried to 24 hrs, allowing more appropriate transfusions decisions to be made.